a) Field of the Invention
The present invention relates to an illumination device to be used as a surface light source having uniform brightness, for example, as a back light for a liquid crystal cell.
b) Description of the Prior Art
As the conventional illumination device for illuminating a plane surface at a uniform brightness over a relatively wide range, the illumination device having the composition shown in FIG. 1 is already known. This illumination device comprises two sets of parallelly arranged illumination systems; each consisting of a fluorescent tube 1 designed as a linear light source and a reflecting mirror 2 which is arranged under the fluorescent tube 1, and has an arcuate sectional shape or a quadratic curve sectional shape and is elongated in the longitudinal direction of the fluorescent tube 1; and a rectangular diffusing plate 3 arranged on the illumination systems.
Since the conventional illumination device of this type uses an arcuate reflecting mirrors having the sectional shape or a quadratic curve sectional shape, the illumination device has large thickness d and is designed as a relatively large unit accordingly. Therefore, the conventional illumination device is not suited for use as an illumination device for back lighting of a liquid crystal cell. Further, the conventional illumination device cannot assure uniform luminance on the diffusing plate 3 and has a defect that luminance is too low or the diffusing plate is too dark. In order to obtain a relatively uniform luminance distribution with this illumination device, it is sufficient to reserve a wide distance between the illumination systems 10 and the diffusing plate 3, but such a corrective measure is undesirable since it inevitably enlarges the illumination device and lowers luminance.
Further, as an illumination device of this type and having small thickness, the illumination device having the composition shown in FIG. 2 is also known. Speaking concretely, this illumination device comprises fluorescent tubes 1 parallelly arranged in a reflecting member 4 which is composed by arranging inclined and elongated plane reflecting mirrors 4b on both sides of a plane reflecting mirror 4a, and a diffusing plate 3 arranged over the fluorescent tubes 1. Though this illumination device can be thin and assure relatively high brightness, luminance distribution on the diffusing plate 3 is not uniform as shown in FIG. 3. In addition, since the boundary portion 4c between the bottom surface 4a and inclined surface 4b is folded, there are formed portions at which luminance is varied relatively abruptly on the diffusing plate 3.